Polychlorinated hydrocarbons, such as carbon tetrachloride, chloroform, trichloroethylene, and tetrachloroethylene have been widely used as chemical intermediates, solvents for dry cleaning of clothing, in degreasing operations, and in a variety of other applications. Chlorinated hydrocarbons are very stable compounds and are relatively toxic at low levels.
Due to this fact, chlorinated hydrocarbons have been accumulating in the environment, particularly in groundwaters. Groundwaters have become contaminated by chlorinated hydrocarbons from sources such as disposal facilities, chemical spills, and leaking underground storage tanks. As analytical detection limits have improved, trace amounts of chlorinated hydrocarbons have been detected in many water supplies, causing public concern. Although the use of chlorinated degreasing solvents was severely curtailed in 1976, their improper storage and uncontrolled disposal practices resulted in significant contamination in groundwater aquifers. Due to their high water solubility (e.g., 1100 mg/L TCE at 25.degree. C.), chlorinated solvents are highly mobile in soils and water aquifers. Additionally, dechlorination of trichloroethylene and perchloroethylene by native microorganisms under reducing conditions may also produce appreciable concentrations of partially reduced products, such as dichloroethylene (DCE) and vinyl chloride (VC), in native aquifers. These products also pose serious health concerns.
To date the most commonly applied treatment scheme for contaminated groundwater has been pump-and-treat. The most practical way for doing this has been to withdraw the contaminated water from a well, volatilize the contaminants in an air stripping tower, and adsorb the vapor phase contaminants onto granular activated carbon (GAC). However, there is growing awareness of the limitations of such pump-and-treat technologies in that contaminated site require treatment often for many decades.
As a result, pollution of water by chlorinated hydrocarbons as become an important environmental problem and contaminated groundwaters represent a large portion of environmental remedial action plans throughout the world.
Chlorinated compounds can be degraded by reductive dechlorination, that is, replacement of chlorine substituents by hydrogen. Metallic elements, such as iron and zinc, have been used to degrade chlorinated organic compounds.
In the patent literature, patents are issued that use metals or metallic couples to degrade chlorinated organic compounds. In U.S. Pat. No. 3,640,821 to K. H. Sweeney and J. R. Fischer, metallic zinc is used for removing pesticides from aqueous solutions. U.S. Pat. No. 3,737,384, also to Sweeney and Fischer, discloses the use of metallic couples, in solutions buffered to near neutral pH for the degradation of pesticides.
More recently, researchers in Japan have reported on the degradation of 1,1,2,2-tetrachloroethane and trichloroethylene in aqueous solution in the presence of iron powder: Senzaki, T. and Y. Kumagai, "Removal of Chlorinated Organic Compounds from Wastewater by Reduction Process: II. Treatment of Trichloroethylene with Iron Powder" Kogyo Yosui, 1989, 369, 19-25. Gillham and O'Hannesin in their article "Metal-Catalyzed Abiotic Degradation of Halogenated Organic Compounds" IAH Conference on Modern Trends in Hydrogeology: Hamilton, Ontario, May 10-13, 1992, have extended the list of chlorinated solvents that can be reduced by iron metal. Recently, Gillham received a U.S. Pat. No. 5,266,213, for his method for cleaning halogenated contaminants from groundwater. The process involves feeding contaminated groundwater through a trench containing a metal such as iron, under strict exclusion of oxygen, and over a lengthy period of time.
Commonly owned and assigned co-pending U.S. patent application Ser. No. 08/318,151, U.S. Pat. No. 5,447,639, issued Sep. 5, 1995, under anaerobic conditions, uses ferrous sulfide to reductively dechlorinate chlorinated hydrocarbons.
The use of a reactive metal or metallic compound, such as iron or zinc, to treat a contaminated groundwater or process stream results in the treated water having a very high pH. pH's in the range 9 to 10 are commonly observed when groundwaters are treated with iron metal in a continuous-flow column owing to the water itself serving as the oxidant. These corrosion processes result in increased pH in weakly buffered systems, such as in carbonated-buffered groundwaters.
Under reducing conditions, the pH increase also favors the formation of iron hydroxide precipitates, i.e., Fe(OH).sub.3 and Fe(OH).sub.2. The formation of a surface layer of precipitates inhibits further dissolution of the iron and inhibits reduction of chlorinated organic compounds by forming a barrier between the reactive iron and the adsorbed chlorinated organic compound.
The oxidation-reduction or redox reaction is controlled by the oxidation potential of the aqueous solution or Eh. A positive Eh indicates that an aqueous phase is oxidizing; a negative value indicates that it is reducing. Although it is difficult to accurately measure in situ Eh in groundwater, there is some evidence to suggest that a range of Eh from -0.2 to +0.7 V can occur. Dissolved oxygen present in aerobic aquifers may be chemically reduced by contact with a variety of reducing species, including reactive metals. This results in the lowering of the Eh of the groundwater and makes reductive dechlorination more favorable.
The reduction process is also a pH-dependent reaction, in which lower pH promotes a faster reaction rate over the pH range 6.0 to 8 relative to the pH range 9 to 10. Hence, a reducing composition or mixture that controls the pH and also lowers the oxidation potential (Eh) of the aqueous phase would greatly improve the process by which chlorinated solutes in water are reductively dechlorinated by contacting them with a reactive metal phase such as iron metal.